1. Field of the Invention
The present invention relates to a slip control system for a motor vehicle and, in particular, to a slip control system which includes an automatic transmission and a traction control system which controls a driving force transmitted to a driving wheel.
2. Description of the Related Art
There has been proposed a slip control system for a motor vehicle as shown in Japanese Patent Laid-Open Publication No. 58-16948, No. 57-22948, No. 62-231836 and the like. When an excessive slip is caused between driving wheels and road surfaces during starting and/or running operations, the slip control system decreases the excessive slip under a predetermined value to obtain driving forces which are transmitted to the road surfaces from the driving wheels and to obtain an optimal friction force between the driving wheels and the road surfaces. As a result, not only the starting and accelerating abilities but also the running and driving stabilities are improved.
The slip control system includes a traction control system. The traction control system includes an engine control system which decreases a degree of an opening of a throttle valve, delays an ignition timing, and cuts the supplying of fuel to the engine so as to decrease an excessive engine output, and/or a brake control system which increases brake fluid pressure so as to increase a braking force of driving wheels transitionally. When the vehicle is running on a low friction coefficient road surface, an excessive depression of the acceleration pedal causes an excessive slip in the driving wheels. At this time, the traction control system obtains a desired friction coefficient between the driving wheels and the road surfaces by decreasing the driving torque transmitted to the driving wheels. As a result, the traction control system decreases the excessive slip under a predetermined desired value.
However, when the driving torque is started to be decreased by the abovementioned slip control system, a responsibility of the driving torque to the accelerator operation is increased. As a result, there is a problem that a driver is not able to operate an accelerator easily.